1. Field of the Invention
The present invention relates to a semiconductor device and particularly to a semiconductor device for power applications.
2. Description of the Background Art
In general, a high-breakdown-voltage power device is required to block current and keep a high voltage when turned OFF and conduct current with a lowest possible loss when turned ON (ON operation), and further desired to switch between ON and OFF with a lowest possible switching loss.
When the high-breakdown-voltage power device switches from ON to OFF, it is indispensable for this power device to block current without leading to breakdown. The IGBT (Insulated Gate Bipolar Transistor) which is a typical high-breakdown-voltage power device has properties including a reverse biased safe operating area (RBSOA) representing a current-voltage area in which current can be blocked in safety in terms of the fact that the high-breakdown-voltage power device does not lead to breakdown.
Factors that cause the reverse biased safe operating area of the IGBT to narrow may include an increased electric field (generation of a high electric field) and current concentration at the boundary between a device-formed region in which devices such as transistor cell are provided and a breakdown-voltage-holding region including a guard ring or the like. Regarding inductive load switching for example, a high-voltage and large-current state occurs in a turn-off process and, depending on blocking conditions, electron-hole pairs are generated due to impact ionization. If they are generated locally, the temperature increases to cause breakdown in some cases. Especially when there is a large amount of a hole current component, the influence of positive space charge causes the electric field to be strengthened and therefore impact ionization is more likely to occur.
In the case of the IGBT, when holes flow from the collector to the emitter in the ON state, the holes partially stay in a region outside the device-formed region and thus hole current components are present in this region. Therefore, in the turn-off process from the ON state to the OFF state, the electric field is further strengthened in the corners of the device-formed region, which causes local impact ionization and may result in breakdown.
Schemes for preventing breakdown at such corners of the device-formed region are disclosed in the documents: Japanese Patent Laying-Open Nos. 06-21358, 2004-158817, and 2000-150859 as well as International Symposium on Power Semiconductor Devices and IC's (ISPSD), p. 353, 1996.